It would help if you could pull some trouble codes with a PSD-compatible reader. If it is an injector issue (as Builttuff mentioned) it could potentially set a P1316 code (IDM/injector codes present) Since this sounds intermittent, and if a 1316 code is spit out, check undervalve cover harnesses and connectors for loose/damage/burned connections, or rubbing wires on pushrods.